If you have more than one website or blog, apply using the "best" one. Once you get accepted for one website you can use adsense on all your other sites and blogs!
Do not apply with unfinished websites (containing pages "in construction", "coming soon"...), although you might be accepted in the program it is useless taking risks as an unfinished website will never get you any money so wait for it to be finished.
Adsense is based on content (text), so keep in mind that a minimum text is required in all your pages (avoid excessive flash and images).
For a website to be accepted it is commonly accepted that a minimum of 20 pages is often enough. For a blog, we think that a 2 months old blog has serious chances provided you post frequently.
Certain contents are prohibited. If you are running a website with pornographic content, do not try to apply for an adsense account. Here is a complete list of what Google does not accept :
Excessive profanity
Violence, racial intolerance, or advocate against any individual, group, or organsation
Hacking/cracking content
Illicit drugs and drug paraphernalia
Pornography, adult, or mature content
Gambling or casino-related content
Excessive advertising
Any other content that promotes illegal activity or infringes on the legal rights of others
Pop-ups, pop-unders or exit windows that interfere with site navigation
Excessive, repetitive, or irrelevant keywords in the content or code of web pages
Deceptive or manipulative content or construction to improve your site's search engine ranking, e.g., your site's PageRank
Incentives (monetary or point-based) to users or third-party beneficiaries for online activity including, but not limited to, clicking on ads or links, performing searches, surfing websites, reading emails, or completing surveys
Sales or promotion of certain weapons, such as firearms, ammunition, balisongs, butterfly knives, and brass knuckles
Sales or promotion of beer or hard alcohol, tobacco or tobacco-related products, prescription drugs, products that are replicas or imitations of designer goods
The program is only accessible to men and women older than 18 years. So if you are younger, you can either wait for some months and keep building your site's content and popularity or use someone else's identity keeping in mind that you could run into truble if your business does particularly well.
Is your website usable ? Does it have a functionning menu, well layed out pages, interesting content... you wouldn't expect advertisers to put their ads on websites you wouldn't trust yourself!